Constructive Algorithms

Codeforces likes a lot of those problems.

Here are some super hard that I found that I did:

• Output the $k$-th permutation of size $n$ with maximum sum, where the sum is defined as $S(p)={\sum }_{1\le l\le r\le n}\min (p_l,p_{l+1},\dots ,p_r)$
• This video has very good explanation https://www.youtube.com/watch?v=6K027clptCc
• basically, the number of subarrays that includes $i$ is given by $i\ast (n-i+1)$ (1-indexed)
• Once you figure out that logic and draw out the numbers, then you see that its always increasing